Tandem-operated echo suppressor



1 B. L. CHILD" TANDEM OPERATED ECHO SUPPRESSOR July 14, 1925.

. I VENTOR A BL. 67am Filed July 2; 1924 ATTORNEY ruemei uly 14, UNITED-STA 'zrmnmr-orm'rnn iicno surransson; Application aim July 2,

To all whom it may concern: Be it known that I, BRADLEY 'residing at Brooklyn, in the county of;

Kingsand-State of New York, have in 6 vented certain Improvements in Tandemfollowing is a specification. r

This invention relates to' t ansm1ss1on-c1rcults, and more particularly to transmlsslon Operated Echo Suppressors, of which the circuits employing echo suppressors.

, One ofthe objects of the invention is'to A provide an arrangement whereby two ormore circuits, each equipped with echo 'sup-.

pressors, may be operated in tandem. -The problem presented when two or. more circuits, each having an echo suppressor, are

switched together, is to prevent the circuits .gettin into sucha condition that, when two subscribers at'the ends begin to talk at about the same time, nei t her subscriber will be able to hear the other.

The nature of-the trouble will be evident if we imagine that two circuits, each contaming an echo suppressor, are connected together. If the subscriber at end A begins scriber at end B hassalready started to talk and the voice currents generated at his sta- .tion have caused the suppressor near his end to operate, itwill'be evident thatthe voice of the subscriber at A will be blocked by the su pressor near'end B while the voice oi the, su criber at B will be blocked by. the sup- 0 pressor near the A end. 1 Talking under these conditions would be diflicult because part of the time both subscribers might be talking with neither one hearing the other. In accordance with the present invention,

.itiszproposed'to overcome thisi-difliculty by arrangingthe echo suppressors so that they, do not absolutely disable the ath transmitting in the opposite direction ut merely creasethe loss in such path. The loss introduced by the operation of the. echo su pressor will reduce the echo efiectand enable the circuit to be operated at a higher-gain than. if the echo suppressor-were not-present, and at'the same time 1.. cam imooxn'nw; New {Your} 85mm; rof Amman AND mnennrn commmr, AJ com'om'rrou or new-roux.

Li CHILD,

11m. semi-1 Kansans. 1

subscriber tohearthe: other when bot suppressors are operated, althou hpinzieach case the'other subscriber will be eardxata very much reducedvolume. i

The invention may-now be understood from the following detailed description,

when read in connection with the accom-nl .panying drawing, the figure of which illustrates a 'tion.

preferred embodiment oithe inven- Referring to the drawing,"threeoiiicegwli,

'Y and Z, are shown with afour-wirev-repeater circuit'extending' from ofliceto 1 office Y, and a similar. four-wire repe'ater circuit extending from oflice ,Y to; oiliceuZ.

The four-wire circuit extending "from office ate point with an echo suppressor, schematically indicated at X--Y and YX -Similarly, the four-wire circuit extending from oflice Y to ofiice Z is provided at an inter- *Xto oilice Y'isprovided at some intermedi v mediate point with a similar echo, ,'sup-- .5

pressor,ischematically indicated at Y.-. -Z and Theseecho suppress ors areofla type wellknown in the art and comprise some jrelay arrangement responsive. to voice currents;

transmitted over one-line of the four-wire circuit to operate a COHtaCtIWhlChdisables the line transmitting in the opposite direction. In echd suppressors, as heretofore op-f erated, it has usually beencustomary to arrange the contact to short-circuit the transmissionpath. In accordance with the present invention, however, the echo suppressor X Y, associated with the ath transinitting from X to Y'between v0 cej Xa'nd ofiice. 1 Y, is arranged so that its contactbridges a resistance element RW acrossjthe line transmitting from Y to'X. 'Likewise, the echo suppressor- Y,- X associated with the line transmitting-- from Y toll, is arranged so" I that in response. to voicecurrents-it shunts a resistance RE across the line transmitting from X to Y. Similarly, theecho suppressorsassociated with the four wire circmt between ;oflice' Y 'and oflice Z are arranged to" bridge shunt redstances RE orRW', as the case may be, across. the linestransmitting "in the opposite directions, respectively. Suppose the twofour-wire circuits are connected through the usual toll 69rd (J at .oflicerY in order to establish a transmission circuit between ofiice X and oflice Z. If the echo suppressors were arranged to directly voice'currents had travelled to the echo suppressor Y--Z, the latter echo. suppressorwould operate to short-circuit the line transmittin from Z to Y- between ofiice Y and oflice Suppose, howe er, before the voice currents transmitted from ofiice X' arrived at the echo suppressor Y-Z, waves from the subscriber at oflice Z should arrive at the echo suppressor ZY. The latter would operate to short-circuit the line transmittinzg from Y to Z between ofiice Y and ofiice This would prevent the voice currents originating atoflice X from being transmitted beyond the echo suppressor ZY to the subscriber at oflice- Z. In the meantime, the

-voice currents originating at ofiice Z would be transmitted throu h the toll cord to the point at which the cc 0 suppressor X-Yis located, but would there be blocked by means of the short-circuit so that they could not be transmitted on to the subscriber at ofiice X.

Neither subscriber would be able to hear the other. A

If, however, resistances are shunted across the lines by the echo suppressors, the transmission in each direction will not be entirely blocked but a transmission loss will be intro duced in the line transmitting-in the opposite direction b the o eration of the echo suppressor. bvious y, if both subscribers should begin talking at once under these conditions, each subscriber would beable to hear the other, although at a considerable reduction in volume, so that one subscriber would cease talking, thereby restoring the path in the-opposite direction to normal. At the same time, considerable advantage in the operation of the circuit would arise by means of the loss introduced by the echo suppressor since the volume of the echoes would be cut down by the amount of the loss inserted.

nected in tandem, a pair "of echo suppressor units associated with each circuit, each' echo suppressor being responsive to currents transmitted in one direction only. and means controlled by the echo suppressor unit in res onse to currents in one direction to intro-- uce a loss inthe path transmitting in the opposite direction, the loss being so proportioned as to prevent the transmission of harmful echo currents while at the same time permittin direct transmission over the paths wit suflicient volume to be heard at the distant end.

2. In a transmission system, a plurality of four-wire circuits arranged. to be connected in tandem, each four-wire circuit comprising a pair of lines for transmitting in opposite directions, an echo suppressor unit associated with each line and responsive to voice currents transmitted over the line with which it is associated, an impedance element associated with the opposite line of each four-wire circuit, and means controlled by the echo suppressor unit associated with one line of a four-wire circuit to shunt said impedance element across the opposite line of the four-wire circuit, said impedance element being so proportioned as to introduce a transmission loss in the line transmitting in the opposite direction sufiiciently great to prevent the transmission of harmful echo currents while permitting the direct transmission of voice currents with sufiicient volume to enable them to be heard at the distant station.

In testimony whereof, I have signed my name to this specification this 1st day of July, 1924.

BRADLEY L. CHILD. 

